MYSQL Average over day statististics - mysql

Question

Use Case: I have a cron checking every 5 minutes some statistics and insert it into the database table stats
**Structure**
`time` as DATETIME (index)
`skey` as VARCHAR(50) (index)
`value` as BIGINT
Primary (time and skey)
Now I want to create a graph to display the daily average in progress over the day - so i.E. a graph for playing users:
from 0-1 i have 10 playing users (avg value from 0-1 is now 10)
from 1-2 i have 6 playing users (avg value is now 8 => (10+6) / 2)
from 2-3 i have 14 playing users (avg value is no 10 => (10+6+14) / 3
and next day it begins from start
I got already queries running, but it takes 3.5+ seconds to run
First attempt:
SELECT *
, (SELECT AVG(value)
FROM stats as b
WHERE b.skey = stats.skey
AND b.time <= stats.time
AND DATE(b.time) = DATE(stats.time))
FROM stats
ORDER
BY stats.time DESC
Second attempt:
SELECT *
, (SELECT AVG(b.value)
FROM stats as b
WHERE b.skey = stats.skey
AND DATE(b.time) = DATE(stats.time)
AND b.time <= stats.time) as avg
FROM stats
WHERE skey = 'playingUsers'
GROUP
BY HOUR(stats.time)
, DATE(stats.time)
First try was to get each entry and calculate the average
Second try was to group by hour (like my example)
Anyway, this does not change anything in performance
Is there anyway to boost performance in mysql or do i have to change the full logic behind it?
DB Fiddle:
https://www.db-fiddle.com/f/krFmR1yPsmnPny2zi5NJGv/4

Answer 1

I suggest to separate the calculation of the average per hour from the calculation of the days average and to calculate these values only once per hour via grouping.
If you are on MySQL 8, I suggest to use CTE as follows:
with HOURLY AS (
SELECT distinct
DATE_,
HOUR_,
AVG(b.value) as avg_per_hour
FROM (SELECT s.value, DATE(s.time) DATE_, HOUR(s.time) HOUR_
FROM stats s
where skey = 'playingUsers'
) b
GROUP BY b.DATE_, b.HOUR_
ORDER BY b.DATE_ DESC, b.HOUR_ DESC
)
SELECT *
, (SELECT AVG(b.avg_per_hour)
FROM HOURLY as b
WHERE b.DATE_ = HOURLY.DATE_
AND b.HOUR_ <= HOURLY.HOUR_) as avg
FROM HOURLY
This statement lasts < 300 ms in the given fiddle.
The calculation corresponds to the algorithm you described in the table above.
However, the results differ from those of the statements presented.